The internet is reasonably open and accessible on iOS, but utilizing the full capability of an iOS device requires the App Store.
Stallman spent a ton of time crusading about how some things are appliances, and other things are computers; those things that are computers should offer flexibility (and ideally openness) in terms of what software you can run on them. In this case, the largest manufacturer of computing devices and software wants 30% of every transaction from native software run on their devices.
Epic was basically looking for preferential treatment, but now they're stepping up to the plate and saying the App Store is not market-friendly. It seems like they could be right, seeing as large as Apple is, and what role they actually have in computing.